The present invention relates to heat exchangers.
Although heat exchangers were developed many decades ago, they continue to be extremely useful in many applications requiring heat transfer. While many improvements to the basic design of heat exchangers have been made over the course of the twentieth century, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes.
One of the most problematic aspects associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger.
One type of heat exchanger which is commonly used in connection with commercial processes is the shell-and-tube exchanger. In exchangers of this type, one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bundle in a serpentine fashion.
Fouling can be decreased through the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. The use of higher fluid velocities can substantially decrease or even eliminate the fouling problem. Unfortunately, sufficiently high fluid velocities needed to substantially decrease fouling are generally unattainable on the shell-side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system because of the baffles. Also, when shell-side fluid flow is in a direction other than in the axial direction and especially when flow is at high velocity, flow-induced tube vibration can become a substantial problem in that various degrees of tube damage may result from the vibration.
Existing shell-and-tube heat exchangers suffer from the fact that xe2x80x9cdead zonesxe2x80x9d and areas of fluid stagnation exist on the shell-side of the exchanger. These dead zones and areas of stagnation generally lead to excessive fouling as well as reduced heat-transfer performance. One particular area of fluid stagnation which exists in conventional shell-and-tube heat exchangers is the area near the tubesheet near the outlet nozzle for the shell-side fluid to exit the heat exchanger. Because of known fluid dynamic behavior, a dead zone or stagnant region tends to form, located in the region between the tubesheet and each nozzle. This area of restricted fluid flow on the shell-side can cause a significant fouling problem in the area of the tubesheet because of the nonexistent or very low fluid velocities in this region. The same problem as described above also exists within the region adjacent to the inlet nozzle.
The fluid flow may be at low velocities in particular areas within the heat exchanger such as in the areas between the entry nozzle and the tubesheet and the exit nozzle and the tubesheet. Various solutions to this problem have been provided in co-pending patent application entitled xe2x80x9cImproved Heat Exchanger with Reduced Foulingxe2x80x9d, U.S. patent application Ser. No. 10/209,082 (U.S. Provisional No. 60/366,776). The solutions provided include the inclusion of a shell extension, a conical connection between the shell and the tubesheet and a conical tubesheet extension; these structural elements may be combined as necessary or as desired in order to address fouling problems.
The above described solutions work well in a great majority of cases but in some applications, particularly where the temperature difference between the shell-side fluid and the tube-side fluid is great, excessive differential thermal expansion of the tubes relative to the shell in the lengthwise direction can occur. Significant structural damage can occur as a result of this tube expansion if the tubesheets are welded to the heat exchanger shell.
Yet another drawback of most prior art heat exchangers is their limited flexibility in terms of the overall process design. For example, in most applications it is desirable for shell-side flow velocity to be the same as or roughly equivalent to the tube-side flow velocity. However, given process flow rate constraints it is often difficult if not impossible to achieve a similarity between shell-side and tube-side flow velocities. This is due to the fixed design of heat exchangers in that there are predetermined cross-sections through which fluid may flow resulting in constrained flow velocities within the heat exchanger given predetermined process flow rates into the heat exchanger.
The present invention comprises a novel heat exchanger configuration which preferably uses the axial flow direction for the shell-side fluid and in which dead zones and areas of stagnation are significantly minimized or eliminated. The heat exchanger of the present invention has the tube in the tube bundle extending between a fixed tubesheet at one end of the exchanger and a floating tubesheet which is preferably located in the return head. The floating tubesheet preferably has a conical shaped extension so that tube surface area exposure in regions of low flow velocities is minimized; a similar conical extension may also be provided on the fixed tubesheet. In one particular embodiment, the heat exchanger includes a central pipe which serves to transport tube-side fluid either from the header to the other end of the heat exchanger or from the end where the return end is located back to the header. The tubesheets and tube bundle can be made so as to be easily removable from the shell for cleaning, inspection and/or maintenance purposes.
The heat exchanger components may be configured in modular assemblies. A significant amount of design flexibility may be obtained by using xe2x80x9coff the shelfxe2x80x9d standardized heat exchangers placed in parallel and/or in series with respect to either or both of the shell-side flow and the tube-side flow. The standard size xe2x80x9coff-the-shelfxe2x80x9d heat exchanger modules are employed to maximize the benefits of the fouling reducing aspects of the present invention and to allow for very significant reductions in design time when preparing to implement processes. Several smaller standard size heat exchangers may be employed in parallel or in series or in both parallel and series to achieve the desired process characteristics including meeting the necessary heat-transfer requirements.
The present invention provides advantages including a significant reduction of dead zones and low-fluid-velocity regions which would otherwise lead to significant fouling problems. The heat exchangers also provide other significant advantages such as permitting the removal of the tube bundle for easy and more effective cleaning, inspection and/or maintenance. They also allow for the avoidance of problems associated with differential thermal expansion of tubes relative to the shell in applications where the difference between tube-side and shell-side fluid temperatures is relatively large.